Technical Abstract:
Amplified hillslope soil loss from rain storms following wildfire results from the evolution of runoff and erosion processes across spatial scales. At point to small-plot scales, soil is detached and transported a short distance by rainsplash and sheetflow. Soil transport by water over larger scales is enhanced by high-velocity concentrated flow. Progressive sediment bulking of overland flow over hillslope scales can result in resource damaging mudflows and debris flows. This evolution of sediment transport at this large scale is well-linked to increased connectivity of runoff sources along a hillslope. We present results from a suite of field studies and well documented post-fire erosion responses that demonstrate evolution of these processes from the small-plot to hillslope scales. Our emphasis is on the connectivity of susceptible surface conditions and the ensuing shift in dominant runoff and erosion processes across spatial scales. By connectivity, we refer to the continuity of runoff-generating bare areas and the convergence of overland flow sources that result in increased cross-scale erosion. Connectivity is well represented by the percentage of bare ground, and its influence on runoff and erosion responses for a given storm is strongly governed by the susceptibility of the bare surface and storm intensity. The susceptibility of the soil surface is defined by burn severity as well as inherent soil properties (i.e., texture/structure, water repellency, erodibility), antecedent soil water content, and topography. Our results are from field studies along the rangeland-xeric forest continuum of the interior western United States, but the overall inferences are applicable for sloping lands in arid- to semi-arid landscapes across the globe.